Increasingly, resources such as heavy crude oils, bitumen, tar sands, shale oils, and hydrocarbons derived from liquefying coal are being utilized as hydrocarbon sources due to decreasing availability of easily accessed light sweet crude oil reservoirs. These resources are disadvantaged relative to light sweet crude oils, containing significant amounts of heavy hydrocarbon fractions such as residue and asphaltenes, and often containing significant amounts of sulfur, nitrogen, metals, poly-aromatic hydrocarbon compounds, and/or naphthenic acids. The disadvantaged crudes typically require a considerable amount of upgrading, for example by cracking, hydrotreating, and fluidized catalytic cracking (FCC), in order to obtain more valuable hydrocarbon products. Cracking other than FCC is generally effected by treating a crude or a heavy fraction of a crude at a temperature ranging from 375° C. to 500° C., optionally in the presence of a catalyst (catalytic cracking) and optionally in the presence of hydrogen (hydrocracking), and results in the decomposition of larger and heavier molecular weight constituents to smaller, lighter molecular weight compounds by cleavage of carbon-carbon linkages. Hydrotreating is generally effected by treating a crude or a fraction of a crude at a temperature ranging from 260° C. to 400° C. in the presence of hydrogen, and optionally in the presence of a catalyst, and results in reduction of sulfur, nitrogen, oxygen, and metals in the crude. FCC is generally effected by treating a heavy hydrotreated, cracked crude fraction having a boiling point of at least 343° C. (650° F.) at a temperature of at least 500° C. with a fluidized cracking catalyst to further crack the hydrocarbons to produce naphtha, kereone, and diesel hydrocarbon fractions.
In conventional processes for upgrading disadvantaged heavy crude oil and bitumen feedstocks, the feedstocks are fractionated by distillation to separate the lightest distillate fractions, containing lower-boiling hydrocarbons, by atmospheric pressure distillation. Heavier fractions containing higher boiling fractions, called vacuum gas oils, are separated by subsequent vacuum distillation. The heaviest fraction called residue or pitch containing the highest boiling, non-distillable hydrocarbons is produced as the bottoms fraction from the vacuum distillation. Lighter hydrocarbons may be recovered from the vacuum gas oils and residue fractions by fluid catalytic cracking or coking. Typically, vacuum gas oils are catalytically cracked in a Fluidized Catalytic Cracker (FCC) to produce lighter hydrocarbons, non-condensable hydrocarbon gases, and coke, where the lighter hydrocarbons may be blended with other distillate fractions to make fuel products. Residue may be cracked in a coker or hydrocracked in a residue hydrocracker to produce lighter hydrocarbon fractions, heavier residue fluid, non-condensable gases, and coke, where the lighter hydrocarbon fractions may be blended with other distillate fractions to make fuel products, and the residue fluid may be further cracked in a Residue Fluidized Catalytic Cracker (RFCC) to produce more light hydrocarbons. The separated fractions may be hydrotreated after atmospheric or vacuum distillation to reduce sulfur, nitrogen, and metals content of the fractions since heteroatoms and metals are undesirable in fuel products produced from the light distillate fractions, and act as hydrocracking catalyst poisons in the vacuum gas oil fraction and residue fraction.
Typically in a conventional process for upgrading a disadvantaged heavy crude feed or bitumen a maximum of about 70%-75% of the carbon content of the disadvantaged crude feed material is captured as non-residue, non-asphaltenic hydrocarbons that are liquid at standard temperature and pressure (STP—25° C., 0.101 MPa), the remainder of the carbon content being produced as gaseous hydrocarbons and carbonaceous solids such as coke. Furthermore, in a conventional process a large percentage of the sulfur is concentrated in high molecular weight refractory heteroatomic hydrocarbons after hydrocracking the hydrotreated heavy fractions, requiring a deep hydrotreatment step to remove of most or all of the sulfur prior to further cracking by FCC so that the sulfur does not poison the FCC catalyst, where a “deep” hydrotreatment step involves hydrotreating at a hydrogen partial pressure of at least 10.3 MPa (1500 psi).
Alternatively, disadvantaged heavy crude oil and bitumen feedstocks may be hydrotreated and catalytically hydrocracked to produce an upgraded hydrocarbon product without initially separating the feedstock into fractions. Current “whole crude” heavy oil or bitumen feedstock upgrading processes also suffer from the production of excess coke and gas, and typically a maximum of about 70%-75% of the carbon content of the disadvantaged crude feed material is captured as non-residue, non-asphaltenic hydrocarbons that are liquid at STP. Current “whole crude” heavy oil or bitumen feedstock upgrading processes also create substantial quantities of refractory sulfur and nitrogen heteroatomic hydrocarbon compounds, which must be removed by deep hydrotreating if the heavy hydrocarbons of the product are to be further cracked by FCC.
Formation of coke and refractory sulfur compounds is a particular problem in upgrading and refining heavy crudes and bitumen, whether as “whole crude” feedstocks or as fractions of a heavy crude or bitumen, that has limited the yield of desirable liquid hydrocarbons from such feedstocks. Cracking or hydrocracking, either thermal or catalytic, is required to obtain a high yield of hydrocarbons that are liquid at STP from a heavy crude or bitumen due to the large quantity of high molecular weight, heavy hydrocarbons such as residue and asphaltenes that are present in such feedstocks. Cracking hydrocarbons involves breaking bonds of the hydrocarbons, particularly carbon-carbon bonds, thereby forming two hydrocarbon radicals for each carbon-carbon bond that is cracked in a hydrocarbon molecule. Numerous reaction paths are available to the cracked hydrocarbon radicals, the most important being: 1) reaction with a hydrogen donor to form a stable hydrocarbon molecule that is smaller in terms of molecular weight than the original hydrocarbon from which it was derived; and 2) reaction with another hydrocarbon or another hydrocarbon radical to form a hydrocarbon molecule larger in terms of molecular weight than the cracked hydrocarbon radical—a process called annealation. The first reaction is desired, it produces hydrocarbons of lower molecular weight than the heavy hydrocarbons contained in the feedstock—and preferably produces naphtha, distillate, or gas oil hydrocarbons. The second reaction is undesired and leads to the production of coke and refractory sulfur-containing heteroatomic hydrocarbons. Furthermore, the second reaction is autocatalytic since the cracked hydrocarbon radicals are reactive with the growing coke particles.
Furthermore, sulfur tends to be concentrated in high molecular weight heteroatomic hydrocarbons in heavy crude oil and bitumen feedstocks. These molecules are also particularly susceptible to annealation due to the large quantity of large, high molecular weight sulfur-containing heteroatomic hydrocarbons in heavy oil and bitumen feedstocks. As a result, significant quantities of large, high molecular weight refractory sulfur-containing heteroatomic hydrocarbons are formed in conventional cracking processes when utilizing a heavy crude oil or bitumen as a feedstock. These sulfur compounds typically poison FCC catalysts that are utilized to further crack heavy hydrocarbons into more desirable light hydrocarbons.
Therefore, hydrocarbon-containing feedstocks having a relatively high concentration of heavy hydrocarbon molecules therein are particularly susceptible to coking and the formation of refractory high molecular weight sulfur-containing heteroatomic hydrocarbons due to the presence of a large quantity of high molecular weight hydrocarbons and heteroatoms in the feedstock with which cracked hydrocarbon radicals may combine to form coke or refractory sulfur-containing hydrocarbons. As a result, yields of non-residue, non-asphaltenic hydrocarbons that are liquid at STP from heavy crude oils and bitumen have been limited by coke formation induced by the cracking reaction itself. A desirable characteristic of coking is that it tends to concentrate large aromatic ring structures, sulfur, nitrogen, and metals in the coke—leaving cracked, lighter hydrocarbon fragments of improved quality relative to the residue fraction of the feedstock—but this concentration effect is obtained at considerable expense in liquid product yield (at STP).
Numerous catalysts have been developed for use in processes for hydroprocessing disadvantaged hydrocarbon feedstocks, either as “whole crude” feeds or as heavy fractions of a heavy crude oil or bitumen, however, such catalysts have not eliminated problems associated with coking and production of refractory sulfur compounds, and catalyst activity may be significantly reduced over time by accumulation of coke on the catalyst. Conventional hydrocracking catalysts are generally selected to possess acidic properties that catalytically facilitate cracking by promoting the formation of cracked radical carbocation hydrocarbon species from hydrocarbons in the feedstock. Such catalysts typically include an acidic support, usually formed of alumina, silica, titania, or alumina-silica, on which a Group VIB metal or metal compound and/or a Group VIII metal or metal compound is deposited or interspersed to catalyze hydrogenation of the cracked radical hydrocarbon species. These catalysts likely promote the formation of coke and refractory sulfur compounds since they induce the formation of highly unstable and highly reactive carbocation radical hydrocarbon species without concomitantly hydrogenating the highly reactive carbocation radical hydrocarbon species as they are formed, thereby permitting a portion of the highly reactive radical hydrocarbon species to react with other hydrocarbons, heteroatomic hydrocarbons, or hydrocarbon radicals to form proto-coke or coke, and/or refractory heteroatomic sulfur-containing hydrocarbons.
Improved processes for processing heavy hydrocarbon-containing feedstocks to produce a lighter hydrocarbon-containing crude product are desirable, particularly in which coke formation is significantly reduced or eliminated, the yield of non-residue, non-asphaltenic hydrocarbons that are liquid at STP is increased so that at least 80%—and more preferably at least 90%—of the carbon content in the feed is captured in non-residue, non-asphaltenic hydrocarbons that are liquid at STP, and in which FCC may be conducted upon a heavy hydrocarbon fraction without the necessity of a deep hydrotreating step.